Modularly structured digital communications systems serve the purpose of connecting terminal equipment having versatile performance features to one another and the purpose of connecting such terminal equipment to communications networks, particularly public networks.
A special edition of "telecom teport", ISDN im Buero, 1985, discloses such a communications system to which analog and digital telephones, telecopiers, multi-functional terminals, work place systems, personal computers, teletex, picture screen text stations and data terminal equipment can be connected. The structure of the system is designed such that traditional devices having analog transmission can still be connected in addition to digital terminals, as well as, providing the capability of networking with other communications systems. The critical component part of this communications system is a central communication computer having a system data base and at least one multi-tasking operating system. A "task" is herein defined as an independent execution unit that is composed of a runnable program as well as of a respective executive environment, for example memory occupation or apparatus allocation. At every point in time, each task has the status of "active", "waiting" or "quiescent". Tasks can be executed in parallel to one another by a multi-tasking operating system, whereby this can be performed both by a plurality of processors working independently of one another as well as by a single processor in a time-division multiplex method. Further critical properties of tasks are that they can communicate with one another by messages and can mutually synchronize with one another.
The actual circuit technology of the known digital communications system is modularly constructed such that the entire system structure is based on function modules. Internal system interfaces thereby provide the connection between the modules. The most critical component part of the communications system is the central communication computer that can also have a multi-processor configuration.
The performance capability of such a digital communications system is critically determined by the software structure of the system. A known prior art communications system has a modularly structured software architecture whose critical component parts are, first, the multi-tasking operating system or the data base and, second, the actual switching software composed of the periphery software, of the line-technology software and of the switching-oriented connecting software. Logical message interfaces are provided between these various software complexes with which the software complexes are distributed onto different control hierarchies in different configuration levels of the communications system. The allocation of sub-complexes of the control software onto the individual software modules or the overall software structure of the known communications system are adapted to a system size of several hundred through several thousand connections.